The girl rushed back into the library and she, too, gave a shriek.

"It's gone!" she cried. "It's gone!"

The boys sprang up the stairs three at a time and into the library. Lucia was right. The box was gone.

"It must be the old boy, sure enough," said Billie, "or at least one of his imps." Then to the maid: "How did he get in?"

"I don't know, senor. I only saw him go out. He went right through the door without opening it."

"Nonsense!" from Donald. "His Satanic majesty might go through a solid door, but the box wouldn't. There is some other explanation."

"But who could it be?" queried Lucia in great fear.

"Some one who knows the house," declared Adrian emphatically. "He may have put on some masquerade costume just to frighten these superst.i.tious servants in case he was discovered."

"I believe Ad is right," agreed Donald. "What do you think, Billie?"

"Maybe, Don; but I have a theory of my own. If I am right, I can work it better if I tell no one."

At this moment the physician entered the room.

"How is Santiago?" asked Donald.

"Better! He has gone to sleep, and if he is not disturbed, I expect him to be greatly improved when he awakens. I should not be surprised if this were the turning point in his illness."

"Then we had all better go down on to the first floor and leave him alone with his nurses."

"I'd give a good deal to know where Strong is," remarked Donald as they were drinking their coffee an hour later.

"So would I," declared Billie. "I believe, if we can find him, we can solve this entire mystery."

"As soon as we finish our coffee," suggested Donald, "let's get busy and find him. He can't be far."

"That's right," said Adrian. "We'll round him up in short order.

Hey, Billie?"

"We ought to; but I'll tell you what. You fellows go out and see if you can get a line on him, and I'll hang round the house to see that the devil doesn't come and steal the rest of the house."

"Especially Lucia," laughed Donald.

"That's all right," was the good-natured reply; "but I have an idea that the devil and Strong may have something in common."

"You don't think it was Strong who came and took the box, do you?" queried Adrian.

"I don't know just what I do think, Ad; but I'm going to do a little detective work and I want to give the impression that we are all out. When you fellows go out, don't say anything that would cause any one in hiding to think we are not all going out together. Do you sabe?"

"Sure. I hope you'll get a clue if we do not."

When the others had gone, Billie sat quietly in his room for a long time. He could easily have gone to sleep, as he had had only a couple of hours' rest, but he made up his mind that he would not be caught napping again if anything should happen.

But nothing happened.

The minutes pa.s.sed into hours and it was rapidly nearing noon when Billie made up his mind that it was a bad job.

"We had our chance at daylight," he muttered, "and now we will not get another. Whoever is after the box, has it and is not going to take any further chance of being caught."

He went out into the patio and looked up at the sun. It was almost in the zenith and the air was stifling.

"Any one would be a fool to go out at this time of day," he mused. "I wonder where the fellows are?"

He stepped back under the shade of the arcade that extended clear around the patio and threw himself onto a stone seat.

"Queer old place," he thought, "and a queer old seat."

He laid his hand on one of the carved arms and mechanically toyed with an eagle's head that formed one of the decorations. To his surprise the head turned in his hand.

"I hope I haven't broken it," he said as he examined it more closely.

It was clearly made to revolve and so he turned it clear around, when of a sudden the arm of the seat fell apart and the bottom collapsed, disclosing to Billie's astonished eyes a pair of stairs.

Almost thrown to the pavement by the giving way of the seat, Billie picked himself up and looked about to see if he were observed.

There was no one in sight and he stooped down and examined the stairs carefully. Then he straightened up and rubbed his chin as a sudden gleam of intelligence pa.s.sed through his brain.

"So," he muttered, "this is why Santiago dwells in a house that is directly back of the banker's. That was his box and he is the strange man who made the million-dollar deposit in Don Esteban's bank."

Then he stopped and pondered.

"But who stole the box? Not Santiago, for he has not left his room for days. If it was Strong, he must have entered the bank from some direction other than this. I don't understand, but I'm going to make some further investigation."

He reentered his room, buckled on his automatic and took from his suit case--which, by the way, he had located at the railway station along with that of his companions after the occupation of the city by the marines--his electric torch. Then he went out and descended the stairs, which he discovered were twenty-four in number.

Reaching the bottom he found himself upon a landing some six or eight feet square, from the opposite side of which another flight of stairs ascended.

"I reckon I better see where they lead to," was his comment as he slowly began their ascent.

After going up eighteen stairs his head touched the floor above.

He counted the remaining steps by the light of his torch and found that there were six more. This would make the floor over his head on an exact level with the floor of Santiago's house.

"It's just as I expected," he muttered. "If I can get through this floor, I shall find myself in Don Esteban's residence--somewhere. But just where? That's the question."

He pushed upon the stone above his head, but it refused to move.

Then he held up his torch and examined the ceiling carefully.

Whatever the method by which the stones could be moved, it was carefully concealed.

Much chagrined, Billie at length decided to retrace his steps and await the coming of his companions. He reached the landing and crossed over to the steps by which he had descended.

Glancing up, he uttered an exclamation of surprise, for there at the opening and peering down the stairs was Ambrosio, the ape.